As technological development of various devices and demand therefor continue to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries which have high energy density and voltage and exhibit long lifespan and low self-discharge rate are commercially available and widely used.
In such lithium secondary batteries, a lithium-containing cobalt oxide (LiCoO2) with a layered crystal structure, LiMnO2 with a layered crystal structure, a lithium-containing manganese oxide such as LiMn2O4 with a spinel crystal structure, or a lithium-containing nickel oxide such as LiNiO2 is generally used as a positive electrode active material. In addition, a carbon-based material is mainly used as a negative electrode active material thereof. Recently, demand for high-capacity secondary batteries is increasing, and thus, use of a silicon-based material or a mixture including a silicon oxide-based material having effective capacity 10-folds or higher than that of a carbon-based material is under consideration.
However, lithium secondary batteries have various problems. For example, some lithium secondary batteries have problems related with characteristics in manufacturing and operating a negative electrode.
For example, in an initial charge and discharge process (activation process) of a carbon-based negative electrode active material, a solid electrolyte interface (SEI) layer is formed on a surface of the negative electrode active material and, accordingly, initial irreversibility is induced. In addition, an SEI layer is collapsed in a continuous charge and discharge process and an electrolyte solution is depleted in a regeneration process, whereby a battery capacity is reduced.
Furthermore, in the case of a silicon-based material, high capacity is exhibited, but a volumetric expansion ratio thereof is 300% or more with increasing cycle count. Accordingly, resistance and side reaction of an electrolyte solution may be increased, and thus, problems due to SEI layer formation, such as electrode structure damage, may be intensified.
Since a silicon oxide-based material has a lower volumetric expansion ratio and superior fatigue life characteristics, compared to a silicon-based material, application thereof may be considered. However, the silicon oxide-based material also has problems such as SEI layer formation during charging and a high frequency of initial irreversibility to Li2O due to oxygen present in an active material.
So as to address such a problem, research into a method to change oxygen, which is present in a silicon oxide-based material, causing high irreversible capacity into a lithium oxide through pre-lithiation of the silicon oxide-based material is actively underway. Such a method can increase lifespan by reducing initial irreversibility of the silicon oxide-based material. However, when this method is used, large amounts of by-products are generated in a process in which a silicon oxide-based material is pre-lithiated with a lithium source and most lithium oxide is generated on a surface of the silicon oxide-based material. Therefore, there is limitation in reducing irreversibility.
Recently, so as to address such problems, attempts to completely decrease initial irreversibility and, thus, enhance cycle characteristics by feeding a negative electrode into a solution containing a lithium source, applying current to the negative electrode, and, accordingly, allowing pre-lithiation reaction have been made. However, when a lithium layer is formed on the negative electrode through application of current, a lithium by-product is also formed at an active material-non-coated portion of the negative electrode, which was not coated with a negative electrode active material. Accordingly, it is difficult to weld the active material-non-coated portion of the negative electrode to a negative electrode lead, whereby it is impossible to manufacture a cell.
Therefore, there is an urgent need for technology to manufacture a secondary battery with high energy density while addressing such problems.